Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags. In a Modulated BackScatter (MBS) RFID system, the Interrogator communicates to the Tags using modulated radio signals. The Tags communicate with the Interrogator by reflecting (backscattering) back to the Interrogator a Continuous Wave (CW) carrier signal originally transmitted by the Interrogator. Communication originating from the Interrogator and transmitted to the Tag is called the Downlink. Communications from the Tag to the Interrogator is called the Uplink. The Tag has an antenna which it uses to receive messages from the Interrogator and to reflect messages back to the Interrogator. After transmitting a message to the Tag, the Interrogator transmits the CW carrier signal to the Tag. The Tag uses the CW carrier signal to respond to the Interrogator message by reflecting or backscattering the CW carrier signal with the modulated subcarrier signal. The CW carrier signal is typically a signal in the microwave frequency range. The reflected or backscattered CW carrier signal is modulated by the Tag with a modulating signal that is generated at the Tag.
In most RFID systems using MBS, the Tag antenna radar cross-section, i.e., the effective reflective area of the antenna, is modulated by alternately biasing ON and OFF a high-quality microwave diode attached to the antenna. When the diode is in its conducting state, i.e., the diode is biased in the ON state, the diode acts as an RF short circuit. When in the OFF state, the diode acts as an RF open circuit. In this manner the impedance or reflectivity of the antenna is modulated which causes the antenna to alternately reflect and absorb the impinging CW carrier signal. Connected to the correct location on the antenna, such as a feedpoint of an antenna, the conducting diode changes a tuned antenna into a detuned antenna causing the antenna to reflect rather than absorb the impinging CW RF signal.
A modulated backscatter RFID tag is sometimes referred to as a passive radio tag in that it reflects and modulates a continuous wave (CW) radio signal with which it is illuminated in an analogous way to a mirror which can be used for communicating by flashing the light from the sun to an observer at a distance. FIG. 1 illustrates a typical read-only modulated backscatter RFID system comprising an Interrogator 105 and a Tag 101. Tag 101 comprises a microprocessor 102, a modulator 103 and an antenna 104. Within the Interrogator 105, the Reader RF Unit 108 generates and transmits a CW signal by means of the Transmit Antenna 106. This signal illuminates the Tag Antenna 104 which reflects (or backscatters) a portion of the incident signal. The reflection coefficient or reflectivity of the antenna, a parameter well known to those in the field of antenna design which indicates an antenna's ability to reflect energy within a certain frequency range, is modulated directly by a modulating signal. The reflection coefficient of the Tag Antenna 104 is switched between being a good absorber of energy to a good reflector in a time-varying pattern (modulating signal) determined by the data stored in the tag memory. The variable reflection coefficient of the Tag Antenna 104 modulates the backscattered RF signal. This backscattered signal is captured by the Interrogator Receive Antenna 107, amplified and down-converted by the Reader RF Unit 108 and the data recovered by the Demodulator 109, Digital Signal Processor (DSP 110 if present) and Microprocessor (.mu.P) 111.
One of the more important factors that affect directly the overall sensitivity of the RFID System is the efficiency with which the Tag Antenna 104 modulates the reflected RF signal. This, in turn, depends upon the Tag Antenna 104 radar cross-section the impedance matching between the Tag Antenna 104, the Tag Modulator 103, and the quality of the Tag Modulator 103. The Tag Modulator 103, for optimum system performance, is usually a high-quality microwave semiconductor, often a switching diode. For an inexpensive Tag 101, this device alone can constitute a major part of the total Tag cost. Thus, in order to significantly reduce the cost of manufacture of the Tag, the cost of Modulator 103 must be reduced while still maintaining acceptable system sensitivity and range. There are RFID system applications in which high sensitivity and long range are less important than low cost. One such class of applications includes tracking airline luggage. Therefore, there is a need to design a relatively inexpensive Tag for such systems.